There is a need for more effective vaccine adjuvants. Historically alum has been the only approved adjuvant for human use and has been effective and safe when administered with whole-cell or virus-based vaccines. However increased recognition of bacterial pathogenic mechanisms has led to the development of newer vaccines to pathogens that contain a more defined, microbial component selective composition that often is less immunogenic. Simultaneously, adjuvant research has advanced with the identification of monophosphoryl lipid A (MPL), a vaccine adjuvant that can safely boost the immune response. MPL was obtained by selective structural degradation of toxic bacterial lipopolysaccharide (LPS), often referred to as endotoxin. MPL, as a modified LPS, retained its immunogenic characteristics while significantly reducing its toxic effects. However, LPS obtained from several species of anaerobic gram negative bacteria have been shown to contain a naturally occurring low toxicity lipid A. Recently, we demonstrated that two naturally occurring low toxicity lipid A's obtained from Porphyromonas gingivalis can boost the immune response and are effective vaccine adjuvants in two different tumor models in mice (Porphyromonas gingivalis 1435/1449 LPS as an immune modulator, Patent US2007/0134170A1). In this proposal our hypothesis is: Naturally occurring low toxicity lipid A's represent a new class of vaccine adjuvants. We will test this hypothesis by isolating and characterizing low toxicity lipid A's (LT lipid A) from several species of anaerobic gram negative bacteria (Aim 1). We will then examine their adjuvant potential in in vitro (Aim 2) and mouse models of non specific (Aim 3) and specific immunity (Aim 4). These studies will determine if naturally occurring LT lipid A's represent a new class of safe and effective vaccine adjuvants.